. Elements of human physiology. Physiology. APPENDIX 705 its former one. In practice a pair of such magnetic needles are usually employed, arranged reversely, as in Fig. 322, where a and n represent the north-seeking ends of the magnets a h and s n. On arranging them in this way the effect of the earth's magnetism on them is weakened or annulled, and they are spoken of as astatic. A bar compass is suspended over the galvanometer, by the adjustment of which the force with which the suspended pair of magnets tends to ' set' in one direction may be increased or dimi- nished. In Thomson's galvanom
. Elements of human physiology. Physiology. APPENDIX 705 its former one. In practice a pair of such magnetic needles are usually employed, arranged reversely, as in Fig. 322, where a and n represent the north-seeking ends of the magnets a h and s n. On arranging them in this way the effect of the earth's magnetism on them is weakened or annulled, and they are spoken of as astatic. A bar compass is suspended over the galvanometer, by the adjustment of which the force with which the suspended pair of magnets tends to ' set' in one direction may be increased or dimi- nished. In Thomson's galvanometer, as it is used for nerve work, the coils of wire have a resistance of 5,000 to 20,000 ohms. Between the two magnets is fixed a small mirror. The swing of the magnet is recorded by observing the Fig. (3l>' Diagram of galvanometer, with astatic pair of magnets. The arrows show direction of current from battery, and the swing of the needles. excursion of a spot of light reflected by this mirror on to a horizontal graduated scale. In thermo-electric experiments, when very small differ- ences of potential have to be measured through a small resistance, the galvanometer must also have a small resist- ance, which here should not be more than one ohm. The capillary electrometer is an instrument for record- ing and measuring difference of potential. That is to say, if connected with two points, it measures the force which would make a current flow between these two points if they were connected with a wire. Its structure is very simple. It consists of a glass tube drawn out to a fine capillary point. This tube with the capillary is filled with mercury. The pomt dips into a wide tube containing dilate sulphuric acid, at the bottom of which is a little mercury. Two platinum wires melted into the 45. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may
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